The present invention relates to a high bending strength, large impact strength hydraulic substance reinforced with high tenacity acrylonitrile (hereinafter abbreviated as AN) fibers, particularly AN fibers which are obtained by dry-jet wet spinning of an AN polymer having a high degree of polymerization and which have an extremely high mechanical strength, a remarkable smoothness at the surface thereof and a dense and homogeneous fiber structure, as compared to conventional and commercially available AN fibers, and a process for production of the reinforced hydraulic substance.
Asbestos has been hitherto used as a reinforcement for hydraulic substances such as cement or gypsum. Methods for producing asbestos-reinforced plane plates, corrugated plate slates, etc. include wet paper-making methods such as the cylinder mould type and fourdrinier type. A cylinder mould type paper-making method called the Hatcheck's method has acquired a reputation as a preferable method. The asbestos fibers for reinforcing cement have a very good affinity and a strong adhesion force to a cement matrix. The presence of long fibers with short fibers in an approximately mixed state enhances the efficiency of paper-making and makes the reinforcing effect satisfactory. Therefore, asbestos fibers are ideal fibers for use in cement reinforcement.
However, due to the recently revealed harmful nature of asbestos fibers, handling becomes a problem on a world-wide basis. Therefore, the development of substitute materials for use as an alternative to asbestos has begun. As substitute materials for asbestos fibers, inorganic and organic fibers such as glass, polyethylene, polypropylene, nylon, polyacryl, polyvinyl alcohol, carbon, aramide, alumina fibers, etc., have been proposed.
In order to obtain a cement plate having an excellent reinforcing effect and durability in a paper-making method, the following requirements should be met, namely:
(1) fibers having a small diameter should be separated from each other without being entangled and uniformly dispersed in a cement suspension (slurry); PA1 (2) the affinity of the fibers to cement, a factor which greatly affects the paper-making property and the reinforcing effect must be good, and adhesion between fibers and cement must be strong; and PA1 (3) durability, particularly alkali resistance, must be excellent.
However, the organic and inorganic fibers which have been attempted to be utilized heretofore do not satisfy all the above described requirements.
For example, fibers such as polyethylene, polypropylene, nylon, etc., have poor strength and poor modulus of elasticity. Further, their adhesive force to a cement matrix is weak. Thus, a satisfactory reinforcing effect cannot be obtained. Glass fibers have poor alkali resistance and an unsatisfactory adhesion force. Further, aramide fibers and carbon fibers have poor dispersibility, weak adhesion force and are costly. Therefore, these fibers have not yet been adopted. Further, polyvinyl alcohol fibers and acrylic fibers promise a bright future as a replacement for asbestos because alkali resistance is good and adhesion to a cement matrix is strong. However, polyvinyl alcohol fibers encounter a problem in cost. Acrylic fibers have poor tensile strength and a poor initial modulus of elasticity; accordingly, hydraulic substances having high efficiencies such as slates having a high bending strength, etc., cannot be obtained from these fibers.
On the other hand, various methods for improving efficiencies, e.g., bending strength, etc., of hydraulic substances reinforced with the aforesaid acrylic fibers have been proposed in recent years. For example, in Japanese Published Unexamined Patent Application No. 170869/82, there is disclosed a hydraulic substance reinforced with acrylic fibers which contain 98 to 100% of acrylonitrile and have a tensile strength of at least 50 CN/tex (5.65 g/d) and a tensile elongation of at most 15%.
However, the acrylic fibers concretely described in the above-cited publication are all obtained by subjecting acrylonitrile type polymers to wet spinning. The thus obtained fibers merely possess a tensile strength of at most 85 CN/tex (9.63 g/d) and a modulus of elasticity of at most 1510 CN/tex (171.1 g/d). The bending strength of cement reinforced with these acrylic fibers is considerably inferior to that of conventional asbestos-reinforced cement. In addition, the bending strength of cement reinforced with acrylic fibers showing the highest tensile strength of 9.63 g/d is not large and a considerably low bending strength is obtained.
It is generally observed that the impact strength of hydraulic substances which are reinforced with synthetic fibers such as acrylic fibers, etc., described above is improved. However, the improvement is unsatisfactory. It has thus been desired to develop hydraulic substances having not only an improved bending strength but also a high impact strength.
On the other hand, various methods for enhancing cohesion between fibers and cement particles in a slurry state of cement and improving a paper-making property have also been proposed. For example, in Japanese Patent Application KOKAI Publication No. 63833/80, there is disclosed a process for producing a cement plate which comprises adding 10 to 800 ppm of a flocculant selected from strong anionic, medium cationic and weakly cationic flocculant to a cement slurry having formulated no asbestos fibers and then subjecting a cement plate to paper-making.
However, cohesion between fibers and cement particles is hardly achieved and the fibers are easily separated from the cement particles by a shearing force at a paper-making step to cause heterogeneity of the slurry. While an improvement in paper-making efficiency is expected to a certain extent in this process, because flocs of cement particles are formed by the addition of the flocculant and thus, outflow of cement particles from a wire cylinder is prevented at the paper-making step, no improvement in properties of the thus obtained cement plate after molding can be expected.
Furthermore, a process for reinforcement of cement materials using polyvinyl alcohol synthetic fibers having coated thereon 0.01 to 3 wt % of anionic and/or nonionic surfactants and a cationic oil is disclosed in Japanese Patent Application KOKAI Publication No. 134553/81. In Japanese Patent Application KOKAI Publication No. 134554/81, a process which comprises using a cationic oil and a nonionic or amphoteric surfactant in combination as a treating agent is disclosed.
However, when these processes are used, they encounter a drawback due to unsatisfactory paper-making efficiency. Namely, due to unsatisfactory cohesion between fibers and cement particles in a slurry state, paper-making cannot be performed efficiently. The effect of improving adhesion between fibers and cement particles in cement articles, that is, the effect of improving bending strength, can be expected to a certain extent.
As described above, in the case of preparing cement plates through a paper-making process, it is important that cement particles be firmly fixed to the surfaces of fibers in large quantities at a paper-making step, a uniformly dispersed slurry be obtained and, at the same time, a felt material having good uniformity be formed by a cylinder mould machine or a fourdrinier machine while maintaining the firmly fixed state and the uniformly dispersed state. In the prior art processes, however, cohesion between fibers and cement particles in a slurry state or dispersibility of the slurry is insufficient so that paper-making efficiency is not always satisfactory. As a result, it has not been possible to improve the properties of a cement plate.
As a result of extensive investigations in hydraulic substances, the present inventors have discovered the present invention.
An object of the present invention is to provide hydraulic substances having excellent bending strength and excellent resistance to impact, at a low cost, and a process for production of such hydraulic substances with extremely good paper-making efficiency.